docs: changelog and design status for range-read milestone M1 (part 1)

Lists the parsers now reading through Storage, what still needs the
whole file (v2 B-tree-indexed structures: a clean error; raw data: M2),
the equivalence harness, and the evidence that nothing changed: existing
tests, a byte-identical conformance results.json and per-file probe
output against f2ff2c4, and identical slice-API transcripts over 748
files between the two builds.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -15,6 +15,49 @@
   fast path; implemented for `[u8]`, `Vec<u8>`, and references, `Box`es
   and (with `std`) `Arc`s of a `Storage`. Slices and `Vec`s serve borrowed
   bytes, so parsing an in-memory file costs no copy.
+- **The metadata parsers read through `Storage`.** Each converted parser has
+  an `*_in(&dyn Storage, ..)` core, and its `&[u8]` function is now a thin
+  wrapper over it, so no caller changes: the superblock
+  (`Superblock::parse_in`), its extension and cache image
+  (`read_superblock_extension_in`, `cache_image_state_in`), object headers
+  with their continuation chunks (`ObjectHeader::parse_in`), local and
+  global heaps, symbol-table nodes and the group B-tree (v1), fractal heaps,
+  fixed and extensible array chunk indexes, shared messages and the SOHM
+  table (`message_data_in`, `message_data_with_sohm_in`,
+  `load_sohm_table_in`, …), attributes (`extract_attributes_full_in`,
+  `extract_attributes_tolerant_in`, `AttributeMessage::parse_in_storage`),
+  fill values (`dataset_fill_value_from_storage`) and virtual-dataset
+  mappings (`DataLayout::resolve_vds_mappings_in`); also
+  `signature::find_signature_in`. Each structure is read with bounded reads
+  (a prefix, then the structure) instead of slicing the whole file; the
+  open-ended `&file_data[addr..]` slices in these modules are gone. Bounds
+  errors keep their values (absolute position, file length).
+- Structures still indexed by a v2 B-tree — dense attribute storage, a SOHM
+  B-tree index and huge fractal-heap objects found through their B-tree —
+  are not converted yet (the
+  v2 B-tree and dense groups come with milestone M3); over a backend without
+  the whole file in memory they are the clean `ContiguousStorageRequired`
+  error, never a partial result. Raw data, chunk B-tree (v1) indexes and VL
+  data are milestone M2.
+- **No behaviour change**, checked three ways (2026-09-26, tank): every
+  existing test passes unchanged; the conformance sweep
+  (`conformance/run.sh --no-fetch`) gives a byte-identical `results.json`
+  and identical per-file probe output for all 697 files at `f2ff2c4` and on
+  this branch; and a transcript of every converted `&[u8]` function's
+  result over the fixtures, the conformance corpus and the h5py-written
+  files below (748 files, 7 603 object headers) is byte-identical between
+  the two builds.
+- New equivalence harness `clawhdf5-format/tests/storage_equivalence.rs`:
+  every converted parser runs over the file as a slice and over
+  `storage::CountingStorage` — a `Storage` that serves an in-memory buffer
+  through `read_at` only (`as_contiguous()` is `None`), copying what it
+  serves and counting reads — and must give identical results. It walks
+  the fixtures, files h5py writes for it (extensible arrays with super
+  blocks and paged data blocks, paged fixed arrays, large v1 and dense
+  groups, a user block, SOHM list and B-tree indexes, dense, shared and
+  committed-type attributes), and with `CLAWHDF5_STORAGE_CORPUS=<dir>` a
+  corpus (all 653 HDF5 files of the conformance corpus pass). Milestones M2
+  and M3 extend it.
 
 ### Chunked full reads (2026-09-26)
 - **Chunks are decoded straight into the output, into reused buffers.** A

docs/design/range-reads.md

@@ -1,7 +1,9 @@
 # Design: range reads (reading HDF5 without holding the whole file)
 
-Status: proposal, 2026-09-26. No library code has changed; this document is
-the plan for Phase 3's largest architectural change. Every count below was
+Status: proposal, 2026-09-26; the plan for Phase 3's largest architectural
+change. Progress: M1, first part (the `Storage` trait and the metadata
+parsers listed in `CHANGELOG.md` under "Range reads, milestone M1") is done;
+group B-tree v2 lookups, dense groups and the facade are not converted yet. Every count below was
 taken on `tank` on 2026-09-26 at commit `de2a53f`, with the commands given
 next to it. No timing numbers appear here on purpose: the machine was shared
 with other build jobs when this was written.